1. Field of the Invention
The present invention relates to a sheet conveying apparatus and an image forming apparatus, and more particularly, to a pressure mechanism for a pair of rollers for conveying a sheet.
2. Description of the Related Art
In an electrophotographic image forming apparatus or an electrostatic image forming apparatus that is well known to those-skilled in the art recently, when an electrostatic latent image corresponding to an image signal is formed on a uniformly charged photoreceptor as a carrier of the latent image, the electrostatic latent image is visualized by a toner supplied from a developing unit.
A toner image carried on the photoreceptor is transferred onto a recording sheet as a recording medium, and fixed on a sheet P by melting and impregnating the toner with heat and a pressure provided from a fixing device. The fixing device disclosed in Japanese Patent No. 3243143 employs the heat roller fixing method. The fixing device includes a heat roller as a fixing member and a pressure roller located in front of the heat roller. The sheet P is held between the heat roller and the pressure roller, and conveyed by a rotation of the rollers.
The heat roller fixing method disclosed in Japanese Patent Application Laid-open No. S59-67568 and Japanese Patent Application Laid-open No. S62-164077 includes forming a nip portion where the sheet P is held between the rollers. In the nip portion, the heat roller heats the sheet P being conveyed. Therefore, the nip portion is required for providing the heat efficiently, and the rollers forming the nip portion are to be contacted each other with a strong pressure.
The fixing device disclosed in Japanese Patent Application Laid-open No. S59-67568 further includes a pressure lever as an oscillating member that can contact a bearing of the pressure roller, an oscillating lever supported on the same shaft of the pressure lever such that the oscillating lever and the pressure lever pull each other via a spring coupled thereto, and an oscillatable release lever that is arranged in the opposite side to an oscillating edge of the oscillating lever and includes a hook unit removably attached to a portion of the oscillating edge. When the oscillating edge of the oscillating lever is displaced according to an oscillating position of the release lever, the pressure lever moves away from the bearing of the pressure roller via the spring coupled to the oscillating edge, and the pressure roller is set up to be in either the pressurizing condition or the pressure-released condition.
In the fixing device disclosed in Japanese Patent Application Laid-open No. S62-164077, the pressure roller is located on an oscillatable supporting member, and the oscillatable oscillating lever that drives the pressure roller to contact or move away from a fixing roller is located on the oscillating edge side of the supporting member, and an elastic bias spring is attached to near a fulcrum shaft of the oscillating lever. In the configuration above, when a position where the spring is attached is displaced to either one side centering around the fulcrum shaft as a dead point according to an oscillating position of the oscillating lever, the pressure roller is maintained at either a pressurizing position or a pressure-released position against the fixing roller.
In the image forming apparatus including the fixing device disclosed in Japanese Patent Application Laid-open No. H9-101696, Japanese Patent Application Laid-open No. 2003-287973, and Japanese Patent Application Laid-open No. S59-067568, when the sheet P is jammed in the nip portion where the pressure roller contacts and pressurizes the fixing roller, a user can open a side cover of the image forming apparatus including the fixing device and remove the sheet P jammed therein. The fixing device includes a pressure mechanism such that the pressure roller is released to pressurize the fixing roller when the side cover is opened. Therefore, the user can easily remove the sheet P jammed therein.
The detail of the pressure mechanism is described below with assigning reference numerals used in embodiments. FIG. 24A is a diagram for explaining the pressure mechanism of the fixing device disclosed in Japanese Patent Application Laid-open No. S59-67568 when a pressure roller 302 pressurizes a fixing roller 301. When the pressure roller 302 contacts and pressurizes the fixing roller 301 located in front of the pressure roller 302, a bearing 316 of the pressure roller 302 is pressed by a pressure lever 303. The pressure lever 303 is oscillatably supported by a rotating shaft 310 supported by an immovable portion (not shown), and contacts the bearing 316. An oscillating lever 304 and the pressure lever 303 are oscillatably supported by a supporting shaft 307. One end of a spring 305 is attached to a latch position T1 in an oscillating edge 306A of the oscillating lever 304, which is near the supporting shaft 307, by a latch pin 309.
Another end of the spring 305 is attached to a latch position T2 that is an immovable portion. The supporting shaft 307 of the oscillating lever 304 is offset from a center of the spring 305, so that the spring 305 is expanded without interfering the supporting shaft 307.
The oscillating edge 306A of the oscillating lever 304 is divided into a plurality of edges centering around the supporting shaft 307. Namely, the oscillating edge 306A is the so-called “two-forked” as shown in FIG. 24B. The latch pin 309 penetrates through the two-forked oscillating edge 306A and fixes the end of the spring 305 to the latch position T1. A liner portion extending from an oscillating edge 306B to the oscillating edge 306A is pressed and moved by a press roller 313 included in a clamp lever 311. Therefore, an oscillating-lever supporting pin 312 included in the oscillating lever 304 is removably attached to a hook unit 328 located in the front edge of the clamp lever 311.
When the oscillating-lever supporting pin 312 strikes on the pressure lever 303, the oscillating lever 304 is restricted rotating to a position shown in FIG. 24A, so that the oscillating lever 304 can not move to the left side farther on. Therefore, the pressure roller 302 can surely pressurize the fixing roller 301.
In an image forming apparatus according to an embodiment of the present invention shown-in FIG. 1, for example, the clamp lever 311 is integrated with an open-close side cover 202. When the side cover 202 is closed, the oscillating-lever supporting pin 312 is pressed and moved by the press roller 313 located in front of the liner portion between the oscillating edges 306A and 306B, and the oscillating lever 304 is restricted to a displacement. When the side cover 202 is opened, the hook unit 328 located in the front edge of the clamp lever 311 is engaged with the oscillating-lever supporting pin 312, and then the oscillating lever 304 rotates according to the oscillating-lever supporting pin 312 being pulled.
In FIG. 24A, the pressure roller 302 contacts and pressurizes the fixing roller 301 so as to form a nip portion N where an outer peripheral surface of the pressure roller 302 is nipped into an outer peripheral surface of the fixing roller 301 partially. While oscillating the oscillating lever 304, when the both latch positions T1 and T2 of the spring 305 and the supporting shaft 307 of the oscillating lever 304 are aligned, an amount of tension in the spring 305 becomes the maximum amount M1. Namely, the maximum amount of tension in the spring 305 is generated at the time. When the latch position T1 of the spring 305 goes over the dead point, in which the both latch positions T1 and T2 and the supporting shaft 307 are aligned, in any direction, the amount of tension in the spring 305 (M2 in FIG. 25) is smaller than M1 that is the amount of tension in the spring 305 at the dead point. Furthermore, a moment, in which the supporting shaft 307 works as a point of support and M2 works as a point of application, is generated by the traction force caused by a contraction of the spring 305.
When the pressurizing condition of the pressure roller 302 is to be released, the side cover 202 is opened, and the oscillating-lever supporting pin 312 is pulled by the hook unit 328 located in the front edge of a release lever 320. Therefore, the restriction on displacing the oscillating lever 304 is released, and the oscillating lever 304 is allowed to rotate clockwise.
When the both latch positions T1 and T2 of the spring 305 and the supporting shaft 307 of the oscillating lever 304 are aligned, namely, at the dead point, the maximum tension in the spring 305 is generated (see FIG. 24A). As shown in FIG. 25, when the oscillating lever 304 rotates clockwise, the spring 305 is contracted, and the amount of tension in the spring 305 is decreased, and a position (P1) of the supporting shaft 307 is displaced in a counterclockwise direction by the restoring force acting on the pressure roller 302 via the fixing roller 301, and then the pressure lever 303 rotates and moves away from the bearing 316. Thus, the pressure roller 302 is released to pressurize the fixing roller 301.
However, there is a problem in the configuration. The oscillating lever 304 is required to rotate at a relatively large amount of rotation for displacing the rotating shaft 310, but it may be difficult to obtain the sufficient amount of rotation because of interference between peripheral members and the oscillating lever 304 depending on those positions. Therefore, the spring 305 is not contracted sufficiently, and the enough amount of rotation for releasing the pressurization is not obtained.
FIG. 25 is a diagram for explaining the pressure mechanism of the fixing device disclosed in Japanese Patent Application Laid-open No. S59-67568 when the pressure roller 302 is released to pressurize the fixing roller 301. The portions identical to those in FIGS. 24A and 24B are denoted with the same reference numerals and the descriptions of the portions are omitted. When a conveying error of the sheet P, the so-called “paper jam”, occurs, the side cover 202 is opened, and the pressure roller 302 is released to pressurize the fixing roller 301. After fixing the paper jam, it may happen that the side cover 202 is closed even though the oscillating lever 304 is not yet returned back.
When the rotation amount of the oscillating lever 304 increases because of releasing the pressurization of the pressure roller 302, the oscillating-lever supporting pin 312 may move out from a movement path of the hook unit 328 where the oscillating-lever supporting pin 312 is engaged, and further the liner portion between the oscillating edges 306A and 306B may move out from a movement path of the press roller 313.
In this case, it may fail to set up the pressurizing condition of the pressure roller 302, because the liner portion is not pressed by the press roller 313 even though the side cover 202 is closed. Namely, even though the side cover 202 is closed improperly, it may happen that the pressurizing condition of the pressure roller 302 cannot be set up, because the oscillating lever 304 is not returned back. Therefore, when the oscillating edge 306B presses the pressure roller 302, a required pressing force of the press roller 313 is getting increased. Furthermore, a resistance to closing the side cover 202 is also getting increased. Thus, the operability in closing the side cover 202 decreases.